Metal coating baths



United States Patent 2,987,427 METAL COATING BATHS Richard Elliott Shaw,Windsor, England, assignor to Imperial Chemical Industries Limited,London, England, a corporation of Great Britain No Drawing. Filed Jan.27,1958, Ser. No. 711,135 Claims priority, application Great BritainJan. 30, 1957 5 Claims. (Cl. 148-615) This invention relates to improvedacid aqueous baths for coating metal with a tightly adherent layer of asalt, e.g. phosphate, chromate or oxalate, or an oxide.

In particular, the invention is concerned with acid coating baths ofimproved convenience and thermal efliciency which can be used with aminimum consumption of the chemicals therein.

Such salt or oxide coatings are applied to metal, for example, toimprove corrosion resistance and paint adhesion, and to providelubricant or lubricant-carrying layers in drawing processes, and it iswell known that the coatings can be produced by treating the metalsurface with acid solutions. For example, metals may be coated withtightly adherent phosphate coatings by treating them with acid solutionsbased generally on iron, manganese, or zinc phosphates, either togetheror separately. These metals are generally known as coating metals. Thecoating reaction may be accelerated by small additions of more noblemetals such as copper and nickel to the processing solution. Furtheracceleration may be obtained by oxidising agents such as nitrates,nitrites, chlorates, bromates, and certain organic nitro-compounds suchas nitro-guanidine.

It is also well known that phosphate coatings can be produced bysolutions containing metallic phosphates of which the metal does notitself enter the coating. Examples are solutions based on sodium,potassium, ammonium, and magnesium phosphates, which form coatingsconsisting predominantly of phosphates of the metal being treated, e.g.iron phosphates on ferriferous articles.

Oxidising agents such as nitrates, nitrites and chlorates may also bepresent in these solutions.

Similarly, oxalate coatings may be produced by treatment with acidoxalate solutions and oxide coatings by treatment with acid oxidisingsolutions.

A serious disadvantage in the use of acid coating baths of the typementioned above is the relatively high consumption of the chemicalstherein. For example, in producing oxalate coatings on stainless steelor other highly corrosion resistant alloys, the oxalate coating bath isso constituted as to include oxygen bearing sulphur compounds, such asthiosulphates, sulphites, hydrosulphites, thionates and bisulphites,which are capable of yielding sulphur dioxide upon decomposition in acidsolution and the consumption of such sulphur compounds is quite high dueto the escape of sulphur dioxide from the bath to the surroundingatmosphere. This is both wasteful and costly, and, in addition, thesulphur dioxide escaping from the bath is odoriferous and a hazard tohealth. A generally similar problem exists with other types of acidcoating baths, e.g. loss of nitrites from phosphate coating solutions.

A principal object of the present invention is to provide acid coatingbaths, e.g. phosphate, chromate, oxalate or oxide coating baths, andmethods of using same, which make it possible to obtain highly effectivecoatings in a 65 more convenient and etlicient manner and with amaterial reduction in the loss of sulphur dioxide liberating substancesand/ or other chemicals.

Another object of the invention is to provide acid coating baths andmethods of using same whereby objectionable odours and health hazardsare etfectively Additionally, it is an object of the invention PatentedJune 6, 1961 ice to provide acid coating baths which demonstrate a highdegree of thermal etiiciency. It is an additional object of theinvention to provideconcentrates suitable on dilution with water for usein the acid coating baths.

The foregoing objects are realised by providing the acid coating bathwith a non-aqueous surface layer of oil or the like as hereinafterdescribed in detail. The success of the invention is highly unexpected.For one thing, to enable metallic surfaces to be treated satisfactorily,it

has hitherto been customary to conduct a preliminary degreasingoperation and to maintain the surface of the processing bath free fromall oil and grease. If oil has been allowed to remain on the bath theacid solution, being unable to react with the oil coated areas of themetal, has produced patchy coatings. It is, therefore, surprising todiscover that excellent coatings can be obtained on metallic surfacesdipped into certain acid baths covered with a layer of oil.

Furthermore, in the case of, for example, oxalate coating bathscontaining sulphur dioxide liberating compounds, it has been found thata layer of mineral oil or the like effectively reduces the consumptionof these compounds and liberation of objectionable sulphur dioxide fromthe bath even though sulphur dioxide is soluble in the mineral oilsurface layer. This is surprising because it normally would be expectedthat the oil layer would not minimise liberation of sulphur dioxide inview of its solubility in the protective layer. Additionally, it hasbeen found that the present invention when applied to baths forproducing uniform coatings on metal surfaces provides a bath which maybe worked with much less heat loss and less scale formation on heatingsurfaces than has been possible with processes not employing a surfacelayer.

The surface layer material must be one which is liquid at thetemperature of operation of the bath and substantially immiscible withthe acid coating solution. The substance may be, for example, a highboiling point paratiin such as liquid paraflin, petroleum jelly ormineral oil, parafiin wax, an animal or vegetable oil such as lanolin,tallow or castor oil, bitumen, a lower molecular weight synthetic resinsuch as polystyrene.

The layer forming substance must be inert, i.c. stable against reductionor attack by the acid bath or by atmospheric oxygen at the temperatureof the bath. For oxide coating baths non-oxidisable materials should beused. The layer forming substance must also be less dense than thecoating solution.

In practice, it has been found that the depth of the layer can be variedbetween relatively wide limits, these limits being dependent on theviscosity range of the ma terials used, the temperature of operation ofthe bath, the

amount of foam formed at the interface between the inert substance andthe aqueous acid solution, the vigour of any etfervescence in the bathand other operating factors. Desirable results can be obtained using asurface layer having a thickness as low as 3 mm. However, preferably,the layer is at least 10 mm. thick for most efiective results, in, e.g.reduction of acid fumes and spray although a layer thickness as high as50 mm., or even higher, may also be used subject to economicconsiderations.

It is preferable that the layer contains very little or no materialvolatile at the temperature of operation of the bath and it should be ofsuch viscosity at the operating temperature of the bath that the layerwill readily reform after being broken by, for example, the entry orexit of articles to the bath, or the exit of bubbles of gas or vapour.

It is preferred that the layer forming material should be of the lowestviscosity consistent with it being subcentipoises are preferred, careshould be taken that the flash point of the layer forming substance isabove that of the temperature of operation. For this reason, a viscosityrange of from 2-10 centipoises at the operating temperature ispreferred.

The preferred layer forming materials are, at the temperature ofoperation of the bath, liquid hydrocarbon materials, such as liquidparaflin and various grades of oils.

A surprising feature of the invention is that any surface layer whichmay tend to cling to articles entering the bath is readily removable anddoes not seriously affect the uniformity with which the acidic solutionreacts with the metal. In any case, this endency may be minimised byadding one or more surface active agents, preferably non-ionic, to theacid solution. The quantity of surface active agent required isdependent on the working temperature, the duration of treatment, and theamount of gas evolved during the coating reaction. Preferably, thesurface active agent should be present in suflicient quantity to lowerthe surface tension of the bath to less than 40 dynes/cm., desirablyless than 35 dynes.

Cationic surface active agents, in particular quaternary ammoniumcompounds, are suitable but in general nonionie surface active agentsare more effective. Typically suitable non-ionic surface active agentsinclude: ethoxylated dinonyl phenols and lauryl alcohols, ethoxylatedoctyl cresols and cetyl and oleyl alcohols. Anionic surface activeagents, such as sulphonated secondary alcohols, or a sulphatedcetyl/oleyl alcohol mixture may also be used but may necessitate anincrease in the amount of accelerating agent used in the coatingprocess.

There is an advantage in having a surface active agent present in thesurface layer to facilitate the removal of any adherent surface layerwhen subsequently rinsing in water. It is an additional advantage toemploy a surface active agent which is soluble both in the acid bath andin the surface layer so that the one surface active agent fulfills bothfunctions. The surface layer can then act as a reservoir to take up anyexcess of surface active agent which may throw out of solution in thebath at elevated temperatures. Thus the benefits of a surface layer maybe obtained with coating baths which do not cause much gassing of themetal surfaces undergoing treatment. 1

The proportion of surface active agent present in the bath to that inthe layer is thus dependent on the prevailing temperature andequilibrium should be established for optimum results. Experiments wereconducted with surface active agent present in the bath but not in thelayer and panels were diflicult to rinse free from layer material.Panels treated or minutes later were much easier to rinse as surfaceactive agent was taken up into the layer material. Equilibrium wasestablished within about 10 to 30 minutes, usually 30 minutes, and norinsing difliculties were then experienced. Similarly, experiments withthe surface active agent present in the layer material, but not in thebath, gave incomplete removal of layer material which adhered to thepanel on immersion. Panels treated subsequently had all layer materialremoved, and rinsed without difliculty, as the surface active agent wasthen present in both bath and layer. The extent to which the surfaceactive agent is soluble in the bath and layer, respectively, at theoperating temperature is important. Alkyl trimethyl ammonium salts haveadequate water solubility up to 100 C., to prevent the bath beingdenuded by solution into the layer. Some agents may be insufiicientlywater soluble and be preferentially dissolved by the layer which thenadheres towork being processed and may become emulsitied into thetreatment bath. Desirably, the surface active agents used herein shouldjust give a clear solution in the aqueous phase at the temperature ofoperation of the bath and the solubility in the surface layer may varybut 4 should be sufiicient to take up the surface active agent thrownout from the aqueous phase.

It is an advantage to incorporate a dye in the surface layer so that thelayer thickness may be readily observed. The dyed layer can also be usedto warn operators that the bath underneath may be very hot despite theabsence of steam or other fumes. A suitable red dye is Waxoline Red 0.8.(Color Index No. C.I.E. 258 (1924/28l).

As has been stated earlier this invention provides concentratessuitable, on dilution, for use in aqueous baths for coating metal, theseconcentrates comprise an inert liquid and an inorganic layer-formingsubstance.

The concentrates may be liquid, pasty, or solid compositions and theproportion of inert liquid in the concentrate may be varied over a widerange. The actual proportion of inert liquid used in any concentratecomposition will be related to the thickness of the layer required onthe tank in which the concentrate is to beused, and the concentration ofactive ingredients required in the tank, it being desirable to obtain onthe surface of the solution in the tank an inert layer at least 10 mm.thick. Surface active agents may be mixed into the concentrates inamounts appropriate, as detailed earlier.

The invention is illustrated by the following examples.

Example I A phosphate coating solution was made up to the followingcomposition:

Percent by weight The non-ionic surface active agent is an anhydrouscondensation product of a long chain fatty alcohol and ethylene oxide.Specifically, the condensation product of 1 part of cetyl alcohol and 3parts ethylene oxide.

This solution was covered with a 12 mm. layer of a saturated hydrocarbonmaterial having a flash point of 155 C., and a viscosity of 3.5centipoises at the operating temperature containing 0.1% Waxoline" Red0.8. The solution and layer were then heated to C. and maintained atthat temperature for 10 minutes to allow some of the surface activeagent to enter the layer. Cold rolled steel panels were degreased bytreatment in a conventional trichloroethylene degreasing plant and werepassed through the surface layer and left in the phosphatmg solution at90 C., for 5 minutes. The panels were then withdrawn through the surfacelayer, rinsed by im mersion in cold running water, rinsed in hot watercontaining 0.025% chromic acid and 0.025% phosphoric acid, and thendried. The steel surfaces had a typical umform light grey crystallinephosphate coating. These coatings were free from bare patches whichwould be caused by adherent oil and were perfectly normal for use asrustproof coatings or as a base for oil, paint or lacquer coatings.

Example 11 A phosphate coating solution was made up of the followingcomposition:

Percent by weight 1.00

This solution was covered with a 12 mm. layer of the hydrocarbonmaterial used in Example I containing 0.1 Waxoline Red 0.8. The solutionand layer were then heated to 70 C., and maintained at that temperaturefor centipoises at 10 minutes to allow some of the surface active agentto enter the layer. Cold rolled steel panels were degreased andprocessed as in Example I but using phosphating solution temperature of70 C. Again, perfectly normal uniform light grey crystalline phosphatecoatings were obtained, eminently suitable for use under oil, paint orlacquer coatings.

Example III A phosphate coating solution was made up to the followingcomposition:

Percent by weight 08 This solution was covered with the hydrocarbonmaterial used in Example I and heated to 70 C., as in Example II. 0.55%carbon steel wire of A" diameter was pickled in cold hydrochloric acid,rinsed in cold running water and passed through the surface layer andleft in the phosphating solution at 70 C., for 5 minutes. The wire wasthen withdrawn" through the surface layer, rinsed by immersion in coldrunning water, and dried. The wire was coated with a uniform light greycrystalline phosphate coating free from bare patches whichwould becaused by adherent oil and was perfectly normal for drawing to a smallerdiameter using a soap lubricant.

Example IV A phosphate coating solution was made up to the followingcomposition:

Percent by weight 3 Manganese (Mn) This solution was covered with ahydrocarbon material having a flash point of 180 C., and a viscosity of5 4 the operating temperature and heated to 95 C. Cast iron piston ringswere degreased with trichloroethylene and passed through the surfacelayer into the phosphating solution. After 15 minutes treatment theywere withdrawn through the surface layer, rinsed by immersion in coldrunning water, rinsed in hot water, and dried. The piston rings had. atypical iron/manganese phosphate coating, free from bare patches whichwould be caused by adherentoil and were perfectly normalfor anti-wearpurposes, i.e. to hold lubricant and prevent initial wear during therunning-in period in internal combustion engines. Steel panels and nutsand bolts treated in this solution received normal phosphate coatingseminently suitable for rustproof coatings and for sealing in oil, stainor paint.

Example V A phosphate coating bath was made up to the following acomposition:

Percent by weight 0.3

This solution was covered with the hydrocarbon material used in ExampleIV and used at 90 C., as in Example IV. Piston rings, panels, and nutsand bolts, so treated had similar coatings to those in Example IV.

6 Example VI 7 A phosphate coating bath was made up to the followingcomposition:

Percent by weight Manganese (Mn) 0.3

Iron (Fe) 0.1

Phosphate (P0 1.6

, Water remainder A concentrated phosphate coating mixture was made upto the following composition:

Percent by weight Manganese (Mn) 4.0 Iron (Fe) 5 0.25 Phosphate (P0 23.0Nitrate (N0 1.5 Non-ionic surface active agent 1.0

Hydrocarbon material: Flash point C.

and viscosity 112 centipoises at 21 C 16.0 Water Seven gallons of thismixture added to ninety-three gallons of water gave a phosphating bathwith a surface layer to prevent evolution of spray or fumes. The samemixture was used to replenish the bath during use. The surface activeagent was an alkyl phenol condensate with ethylene oxide, namely, 1 partoctyl cresol with 2 parts ethylene oxide.

Example VIII An oxalate coatingibath was made up to the followingcomposition:

Percent by weight Manganese (Mn) 0.4 Nitrate (N0 1.0 Oxalate (COO); 3.5Accelerators 0.3 Non-ionic surface active agent 0.1 Water remainder Thesurface active agent was the same as that used in the preceding example.The accelerators were sodium meta-bisulphite and sodium thiosulphate.

This solution was covered with a 12 mm. layer of hydrocarbon material asused in Example I and heated to 70 C. Stainless steel wire was pickledin a mixture of nitric and hydrofluoric acids, rinsed in water, andpassed through the surface layer into the oxalate bath. After 5 minutesthe wire was removed through the surface layer, rinsed by immersion incold running water, rinsed in hot water, and dried. The wire had atypical oxalate coating, free from bare patches which would be. causedby adherent oil, suitable for anti-wear purposes, i.e. to hold lubricantand prevent wear during drawing-to smaller diameters.

Example IX A phosphate coating bath was made up to the followingcomposition:

remainder remainder I from 'bare patches, suitable to prevent rust creepunder a protective finish in the event of mechanical damage to part ofthe protective finish.

Example X A phosphate coating bath was made up to the followingcomposition:

Percent by weight 0.8

Zinc (Zn) Phosphate (P 1.2 Nitrate (N0 1.3 Iron (Fe) 0.1 Cationicsurface active agent 0.2 Water remainder The cationic surface activeagent was an alkyl trimethyl ammonium salt, specifically, cetyltrimethyl ammonium bromide.

This solution was covered with 12 mm. hydrocarbon material as used inExample I and heated to 85 C. A basket containing small spring steelclips was passed through the surface layer into the phosphatingsolution. After 3 minutes, the clips were withdrawn through the surfacelayer, transferred to a second basket and again immersed in thephosphating solution. This process was repeated to give four immersionperiods of 3 minutes. The clips were then rinsed in cold running water,rinsed by immersion in hot water and dried centrifugally. The clips werethen dipped in spirit stain and oven dried.

By this procedure, the spring clips had received a typical zinc/ironphosphate coating and were adequately protected from corrosion inservice.

Example XI A chromate passivating bath for aluminium was made Thesurface active agent was a sodium salt of perfluoropentane sulphonicacid.

This solution was covered with a 12 mm. layer of hydrocarbon material asused in Example IV and heated to 100 C. Aluminium panels degreased byimmersion for 5 minutes at 98 C., in a 3% meta-silicate solutioncontaining surface active agent, were rinsed in cold water and passedthrough the surface layer into the passivating solution. After 1 hour,the panels were withdrawn through the surface layer and rinsed byimmersion in cold running water, rinsed in hot water and dried. Panelscoated in this way were more resistant to corrosion than untreatedpanels.

The surface layer completely suppressed spray and steam from thesolution during processing without interfering with the coating process.The degreasing solution may also be covered with a 12 mm. layer of thesame oil.

Example XII A chromate passivating bath for magnesium was made up to thefollowing composition:

- Percent by weight Sodium dichromate 8.0 Magnesium sulphate 4.0

8 Manganese sulphate 4.0 Anionic surface active agent (as in Ex. XI) 0.5Water remainder This solution was covered with a 12 mm. layer of ahydrocarbon material as used in Example IV and heated to C. Magnesiumdie-castings were degreased by immersion for 5 minutes at. 98 C., in a3% meta-silicate solution containing surface'active agent, rinsed byimmersiouin cold running water and passed through the surface layer intothe passivating solution. After 30 minutes, they were withdrawn throughthe surface layer, rinsed by immersion in cold running water, rinsed inhot water and dried.

The surface layer completely suppressed evolution of steam and sprayfrom the solution during processing without interfering with thecoating.

Example XIII An oxalate coating solution was made up to the followingcomposition:

Water, to make 1 litre.

An operating bath can be prepared from the foregoing formula byblanketing or covering the same with suflicient commercial white mineraloil to develop a layer approximately 8 mm. thick.

The resulting bath is highly effective for the oxalate coating ofstainless steel. A loss of the chemical components therein is noted bothwhile the bath is in use and while standing. It will be understood fromthe foregoing that other commercial oils and layer depths may also besatisfactorily utilized although for uniform results the minimum layerthickness of 3 mm. mentioned heretofore should be observed.

Other typical and highly satisfactory examples of coating bathsaccording to the invention are noted below:

Example XIV Manganese carbonate, 38% gms 12 Oxalic acid do 15 Nitricacid, 69% ml 24. Sodium thiosulphate ....gms.... 5

Water, to make 1 litre.

Sun oil No. 591 added to obtain a layer depth of 8 mm. over the bath.

Water, to make 1 litre.

Tall oil (purified) added to obtain a layer depth of 8 mm. over thebath.

Example XVI Oxalic acid gms 50 Sodium chloride do 20 Sodium bifiuoridedo 10 Sodium thiosulphate do 2 Water, to make 1 litre.

Lard oil was added to obtain a layer depth of 10 mm.

Example XVII Manganese sulphate, 70% --gms- 15 Oxalic acid (In 20Phosphoric acid, 75 ..ml-- 10 Sodium sulphite gms.... 2

Water, to make 1 litre.

Sufiicient commercial white mineral oil to cover the bath to a depth of10 mm.

Example XVIII Manganese sulphate, 70% grns.. 15 Oxalic acid do 10Phosphoric acid, 75% ml 10 Sodium thiosulphate gms.

Igepal D1710 (Amara Chem. Co.) ethoxylated dinonyl phenol gms- 0.5Water, to make 1 litre.

Ultrol 5 (commercial white mineral oil) added to obtain a layer depth of8 mm. on the bath.

I Example XIX Manganese carbonate, 88% -gms l2 Oxalic acid do 15 Nitricacid, 69% ml 24 Sodium thiosulphate gms.. 5

Atlas Brij 35*(Atlas Powder Co.) ethoxylated lauryl alcohol gms Water,to make 1 litre.

Sun oil No. 598 added to obtain a layer depth of 8 mm. on the bath.

Example XX A manganese phosphate concentrate for phosphating steelconsists of:

A bath 4 ft. x 2 ft. x 2 ft. deep holding 100 gallons requires 100 lbs.manganese dihydrogen phosphate. This is contained in 120 lbs.concentrate together with 18 lbs. hydrocarbon material and 1.2 lbs.surface active agent. 18 lbs. of the hydrocarbon material is 2.1 gallonsand over 8 sq. ft. this gives an inert layer about 15 mm. thick. Thebath was heated to 100 C. and degreased piston rings were inserted.After about 45 minutes the rings were removed and rinsed and it wasnoted that they had received a heavy phosphating coating.

Using comparable formulations, oxalate, chromate, sulphide and othercoating bath concentrates may be prepared for use in the variousprocesses disclosed in this specification.

Example XXI A concentrate composition, suitable, on dilution, forcoating titanium was made up as follows:

A solution was made up as in Example XX using 100 lbs. of the abovecomposition. Titanium wire treated in the solution at 60 C. for 10minutes received a good coating suitable for use as a drawing aid withsoap.

It will be appreciated from the foregoing examples that the inventionmay be used with any of the conventional type acid coating baths, e.g.phosphate, oxalate, chromate, or oxide solutions. Baths modified in themanner described herein may be utilized, as shown, in the treatment ofstainless steel and other metals such as magnesium, aluminium, titanium,zirconium, zinc and cadmium, which are conventionally treated with acidcoating solutions. Usual conditions of temperature and time for coatingthese materials may be utilized. These conditions vary over a relativelywide range depending upon the type of solution and metal being treated.However, generally speaking. treating temperatures of the order of 25 toC., and immersion times of 1 to 60 minutes are utilized.

The eflectiveness of the oil layer in reducing chemical consumption mayreadily be determined by heating baths prepared in the manner-describedin the foregoing examples with and without the layer and checking thechanges which take place in the chemical content of these baths over aperiod of time. Typically, a series of oxalate coating baths having thecomposition of, for instance, Example XV, have been prepared, one of thebaths having no surface-layer and the others having surface layers ofvarying thickness. These baths were then heated to 70 C. Samples ofthese baths were removed periodically and treated with 0.05 N iodinesolution. This demonstrated the effectiveness of the surface layer inretarding sulphur dioxide loss from the baths. Comparative tests of thistype indicate clearly a highly significant reduction in the loss ofsulphur dioxide using oil layers of at least 1 mm. thickness.

Various modifications may be made in the invention described hereinwithout deviating from the scope thereof as set forth in the appendedclaims.

What we claim is:

1. In a process for the acid coating of a metal by immersing said metalin a heated acid coating solution having volatile or decomposablecomponents therein, the improvement whereby loss of components from saidsolution by volatilization and decomposition is minimized, saidimprovement comprising immersing said metal in a hot coating solutioncovered with a non-aqueous surface layer of inert material which isliquid at the operating temperature, said liquid being substantiallyimmiscible with said acid coating solution, substantially non-volatileand stable at the operating temperature of said bath and inert withrespect to the constituents of the bath, both said solution and saidlayer having a surface active agent dissolved therein, the amount ofagent in said solution being sufiicient to lower the surface tensionthereof to less than 40 dynes/cm.

2. The process of claim 1 wherein said solution is an oxalate coatingsolution containing a sulphur dioxide liberating compound.

3. In a process for the acid coating of a metal by immersing said metalin a heated aqueous acid coating solution selected from the groupconsisting of phosphate, oxalate, and chromate coating solutions havingvolatile or decomposable components therein, the improvement whichcomprises initially covering the surface of a heated acid coatingsolution, prior to immersing any metal therein, with a layer ofsaturated hydrocarbon which is liquid at the operating temperature andsubstantially immiscible with said solution, said liquid being stableand substantially non-volatile at the operating temperature of saidsolution and inert with respect to the constituents of said solution,said surface layer having a viscosity of from 2-10 centipoises and aminimum thickness of 3 mm.; including in both the surface layer and saidsolution a non-ionic surface active agent, which is soluble in both saidlayer and solution, the amount of surface active agent dissolved in thesolution being sufiicient to lower the surface tension to less than 40dynes/cm., and give a clear solution at the operating temperature, saidsurface active agent having sufiicient solubility in said surface layerto take up any surface active agent separated out from said solution;and only thereafter immersing the metal through said layer and into saidsolution to prepare said coating thereon.

4. An aqueous acid coating bath consisting essentially of an acidcoating solution covered with a non-aqeous surface layer of inertmaterial which is liquid at operating temperature, said liquid beingsubstantially immiscible with said solution, substantially non-volatileand stable at the operating temperature of said bath and inert withrespect to the constituents thereof, both said solution and said layerhaving a surface active agent dissolved therein,

compound.

References Cited in the file of this patent UNITED STATES PATENTSBurghardt et a1. June 29, 1897 10 Neilson Jan. 7, 1941 Boyle et a1. Dec.8, 1942 Bean -5. Dec. 25, 1945 Douty Mar. 19, 1946 Vance Ian. 3, 1950Douty et al. July 25, 1950 Gibson Dec. 11, 1951 Burnham f May 29, 1956Piccinclli July 23, 1957' Touches Apr. 29, 1958 FOREIGN PATENTS 7 GreatBritain May 12, 1927 France Mar. 23, 1940

1. IN A PROCESS FOR THE ACID COATING OF A METAL BY IMMERSING SAID METALIN A HEATED ACID COATING SOLUTION HAVING VOLATILE OR DECOMPOSABLECOMPONENTS THEREIN, THE IMPROVEMENT WHEREBY LOSS OF COMPONENTS FROM SAIDSOLUTION BY VOLATILIZATION AND DECOMPOSITION IS MINIMIZED, SAIDIMPROVEMENT COMPRISING IMMERSING SAID METAL IN A HOT COATING SOLUTIONCOVERED WITH A NON-AQUEOUS SURFACE LAYER OF INERT MATERIAL WHICH ISLIQUID AT THE OPERATING TEMPERATURE, SAID LIQUID BEING SUBSTANTIALLYIMMISCIBLE WITH SAID ACID COATING SOLUTION, SUBSTANTIALLY NON-VOLATILEAND STABLE AT THE OPERATING TEMPERATURE OF SAID BATH AND INERT WITHRESPECT TO THE CONSTITUENTS OF THE BATH, BOTH SAID SOLUTION AND SAIDLAYER HAVING A SURFACE ACTIVE AGENT DISSOLVED THEREIN, THE AMOUNT OFAGENT IN SAID SOLUTION BEING SUFFICIENT TO LOWER THE SURFACE TENSIONTHEREOF TO LESS THAN 40 DYNES/CM.